This invention is concerned generally with individual control of footage volumes in electronic organs. One particular application of the invention is to organs employing a dual-encoded data format in which a pulse stream is both pulse-position-modulated (PPM) and pulse-amplitude modulated (PAM); i.e. instruments in which a musical note is encoded in pulse position form and the amplitude of the pulse corresponds to the sound volume of the note.
An electronic instrument of this type, specifically an organ, is described in detail in U.S. Pat. No. 3,902,397of Morez and Moore, which is assigned in common with the present application. In general terms, that instrument employs a clock-driven counter and decoder to scan the organ keyboard. Different time slots in the scan cycle represent different musical notes, because the key corresponding to any given note is always interrogated at the same time point in each scan cycle. If the key in question is actuated at the time it is interrogated, a pulse will appear in the scan output at the particular time position which represents the corresponding musical note. Thus, the note selection data derived by scanning the keyboard is pulse-position-modulated.
One of the advantages of this pulse position code format is that the keyboard data pulse train can be enriched with pulses representing various harmonics of the keyboard-selected notes, with but a single key actuation. This is accomplished in the organ of the Morez and Moore patent by passing the scan output, i.e. the keyboard data pulse train, through a shift register (referred to as the pitch shift register). This shift register is in effect a delay line. As each key-actuation pulse passes through successive shift stages, it can be picked off and reinserted in the pulse train at any desired subsequent time slot to represent various harmonics of the original note.
When superimposed upon this pulse-position-modulated scheme, the pulse amplitude technique for encoding of sound volume permits the volumes of the fundamental note and the various selected harmonics to be adjusted independently of each other. Each time a given key-actuation pulse is reinserted into the data train, its amplitude can be increased or decreased as desired. This raises or lowers the sound volume of the particular harmonic represented by that insertion pulse, without in any way affecting, or being affected by, the pulse amplitude (sound volume) of the fundamental note or any of the other pulse insertion harmonics.
In the organ of the Morez and Moore patent, the individual pulse amplitudes (and hence the harmonic sound volumes) are controlled by certain reference voltage levels. The circuit which supplies these volume control voltages is called a drawbar circuit. The particular drawbar circuit in the Morez and Moore patent, illustrated in FIG. 5 thereof, has certain limitations. It offers three sound volume control voltages for each harmonic selected, one of which is adjustable by the musician. The other two are set at the factory, and are not convenient to change. If the adjustable volume setting for one or more harmonics needs to be changed temporarily, there is no way to store and later recall the previous setting. For example, in order to achieve a particular balance of sound volumes among, e.g. nine different harmonics, the musician must balance nine different potentiometer settings. Then if the potentiometer settings are temporarily changed to suit different musical conditions, the labor of resetting the original volume balance must later be performed all over again. In addition, the potentiometer settings are sliding scale adjustments, i.e. analog rather than digital. As a result, it is not possible to know when one has exactly duplicated any previous setting, and the musician must always settle for an approximation.